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Why do sharp ends of blades attract?

  1. May 5, 2013 #1

    PhysicoRaj

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    Hi everyone..
    I was doing some experiments on diffraction and interference. For the single slit I used a shaving razor blade and cut it into half along its length. Then I placed the two halves together forming a slit such that the two sharp edges almost touched each other, similar to the one in the attachment.
    Then I shone my laser and got some diffraction patterns..
    I repeated the experiment with the razor blades, not on any mount, but poked onto a rubber cube so that I could change the aperture during the experiment. As I started moving the blades together, they simply attracted each other and got stuck, leaving no aperture. I tried again, at a particular distance they stick to each other. But with the blunt edges facing each other, this does not happen. It seems they are attracting each other.. why? This seems to be a new phenomenon for me, so let me know what's behind all this..
    Thanks for any replies.
     

    Attached Files:

  2. jcsd
  3. May 5, 2013 #2
    Any chance that the blades are slightly magnetized? The magnetic field would be more concentrated by the sharp edge.
     
  4. May 5, 2013 #3

    PhysicoRaj

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    I dont think so.. it was a new shining blade and was unused. It was kept in the bathroom where no magnetic substances or magnets are kept.....
     
  5. May 5, 2013 #4

    CWatters

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    It might have become magnitised during manufacture (eg sharpening)?
     
  6. May 5, 2013 #5

    HallsofIvy

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    If it was a "new shining blade and was unused" then it is more likely to be slightly magnetised. The sharpening process can align molecules and give a slight magnetism to the blade.
     
  7. May 5, 2013 #6

    Danger

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    Are they close enough together that van der Waals force might play a part?
     
  8. May 5, 2013 #7
    Are they coated in any material which may be magnetized ?? The fact they are attracting (or repelling if the case was made) really suggests magnetization.. Interesting though.. Have you tried with other brands of blade ?
     
  9. May 5, 2013 #8
    Sorry for the naive question, but are you sure they are really attracting each other and it's not the optical effect due to diffraction (as if you look at two fingers approaching each other while in front of the eye)?
     
  10. May 5, 2013 #9
    I took the assumption this was based off an experiment which was contained, also repeated .. Given the layout there are grounds you are right, however its hard to say.The full extent of the experiment would need to be explained. For example was there any liquid in the container for which they transported etc.

    But based off your comment I can take the assumption(although never should be taken) that because it was repeated it was not the diffraction effect. I could very well be wrong!
     
  11. May 5, 2013 #10
    The same way that a microwave oven's mesh screen prevents wavelengths larger than the holes through. The light through the slit will be blocked as soon as the distance between the blades falls below the wavelength of the visible light.
    That would be at about 0.0004 mm.
     
  12. May 5, 2013 #11

    sophiecentaur

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    How much force was involved in this "attraction"? Was it actually measurable?
    If the effect was only 'seen' and not 'felt' then the diffraction explanation seems, to me, most likely. Except that it wasn't reported for a pair of blunt edges. You can get a similar (optical) effect as you bring finger and thumb (very blunt) together. As they are brought closer and closer together, a point is reached where the diffraction effect gives an apparent 'jump' and the gap appears to close up.
    This effect (and also the visible fringes) is difficult to record on a photograph because the aperture of the eye is less than that of a camera. But it does work if you use a pinhole to reduce the aperture.
     
  13. May 5, 2013 #12
    You are offering suggestions based on the light issue. This may have already (and most likely) taken care of.

    (You can get a similar (optical) effect as you bring finger and thumb (very blunt) together.) You can, but demonstrated twice.. In single eye case, this is completely void.
     
  14. May 6, 2013 #13

    NascentOxygen

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    Thought experiments that come to mind:

    Maybe try various half-blade combinations, to see whether there is always attraction, or is there sometimes repulsion?

    Make a sensitive magnetometer and test the blades for N's and S's.

    It may be possible to put them through a demagnetiser, and see whether the effect is diminished? A cheap demagnetiser may not be up to the task, though.

    Perhaps you could slightly magnetise one of them and see whether you can effect repulsion? May need to try this twice, changing the polarity.

    Cut a half-blade in half, trying not to subject it to shock. The pieces should give you a pair of similar magnets. Do these quarter blades still show attraction or do they now repel?
     
  15. May 6, 2013 #14

    Borek

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    Unless you live on the Moon, your bath is right on the magnet surface.
     
  16. May 6, 2013 #15

    sophiecentaur

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    But there appears to be an anomaly, if the experiment was carried out right and if the effect was optical. This anomaly needs an explanation, which could be to do with the observation method. I was not just repeating old ideas.
    Perhps you could read more carefully before complaining so early?
     
  17. May 6, 2013 #16
    Last edited: May 6, 2013
  18. May 6, 2013 #17

    sophiecentaur

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    So far, there are no actual quoted measurements of force so the conversation isn't really going anywhere, is it? If there really is a magnetic force involved then hanging up one blade on a light thread and bringing the other blade close will reveal it. This would mean a result for the thread and put an end to speculation.
     
  19. May 15, 2013 #18

    PhysicoRaj

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    Yeah, I'm sure they attract, because at the time I stop getting any light on the screen, the rubber cubes I poked the blades into, tilt towards the middle.
     
  20. May 15, 2013 #19

    PhysicoRaj

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    This was 'Wilkinson sword stainless steel'...:cool:
     
    Last edited: May 15, 2013
  21. May 15, 2013 #20

    PhysicoRaj

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    AAhhh:bugeye:.. for that matter,.. the blunt ends donot show this effect! Is it because the intensity is more at sharp edges??
     
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